sql_variant

sql_variant is one of the datatypes that I don't often see being used although it has it's advantages. It's kind of like a varchar where you would be able to store numeric characters, alphanumeric characters, dates, ... with the difference that on a sql_variant field you would be able to determine of which datatype the value is.

One tip I would give you is to always explicitly cast the value you insert (or update) to the datatype you want it to be. Apparently SQL Server uses 2 extra bytes to determine the extra information like the datatype etc. (eg. int would be 6 bytes).

The SQL_VARIANT_PROPERTY function gives you information about the variant itself like datatype, total bytes, precision ...

For more information visit:
sql_variant
SQL_VARIANT_PROPERTY